Such a mechanism is provided with a screw comprising an outer thread, with a nut disposed around the screw and comprising an inner thread, and with a plurality of longitudinal rollers that have an outer thread engaged with the outer and inner threads of the screw and of the nut.
In a first type of roller screw mechanism, the threads of the rollers and the thread of the nut have helix angles that are identical to one another and different from that of the thread of the screw, such that, when the screw rotates with respect to the nut, the rollers rotate on themselves and roll around the screw without moving axially inside the nut. The rollers are guided in rotation parallel to the axis of the screw by way of ring gears that are fitted in a non-threaded part of the nut and comprise inner synchronizing gear teeth engaged with outer gear teeth of the rollers. Such a mechanism is referred to as a planetary roller screw.
A second type of roller screw mechanism has a similar operating principle, but differs by way of an inverted disposition. The helix angles of the threads of the rollers, of the screw and of the nut are chosen such that, when the screw rotates with respect to the nut, the rollers rotate on themselves about the screw and move axially in the nut. The rollers are guided in rotation by outer synchronizing gear teeth that are formed on the screw and engage with the gear teeth of the rollers. Such a mechanism is referred to as an inverted roller screw.
With the current design of these mechanisms, during machining operations that are carried out in order to form the outer thread of each roller, the thread-cutting tool cuts the outer gear teeth previously machined at each end of the roller. The thread is thus also present on the gear teeth of each of the rollers, generally in the useful region of the gear teeth that is provided for the transmission of forces and is located between the head radius and the clearance radius of the teeth.
During this transmission of forces, given the presence of these threads on the gear teeth of the rollers, the contact pressure exerted by the gear teeth on the synchronizing gear teeth provided on the nut, or on the screw, is relatively high in the useful region of the gear teeth of the rollers. This causes premature wear to the synchronizing gear teeth.
Furthermore, given the profile in the form of an involute of a circle of the gear teeth of each roller, these gear teeth are conventionally produced by milling. This makes it necessary to provide a relatively large clearance for the axially acting milling cutter axially between each gear teeth of the roller and the outer thread. The axial size of each clearance is directly linked to the radius of the milling cutter. For a given length of the roller, this clearance formed at each end limits the length of the outer thread that can be provided on the roller and thus the loading capacity and the radial and axial rigidities of the associated mechanism. Moreover, with such a profile in the form of an involute of a circle, sharp edges are present on the gear teeth of each roller. In operation, this also helps to create a high contact pressure on the synchronizing gear teeth provided on the nut, or on the screw, in the useful region of the gear teeth of the rollers.